Micro-organisms are archaea, bacteria and some species of eukaryotes

Micro-organisms use a wide variety of substrates for metabolism and produce a range of products from their metabolic pathways.

Micro-organisms are used because of their adaptability, ease of cultivation and speed of growth.

There are variations in what micro-organisms need to grow.

The complex molecules required for biosynthesis are amino acids, vitamins and fatty acids.

Many micro-organisms produce all the complex molecules required for biosynthesis.

Some micro-organisms require the complex molecules required for biosynthesis to be supplied in the growth media.

When culturing micro-organisms, their growth media require raw materials for biosynthesis as well as an energy source.

An energy source is derived either from chemical substrates or from light in photosynthetic micro-organisms.

Culture conditions for micro-organisms include sterility, control of temperature, oxygen levels and pH.

Sterile conditions in fermenters reduce competition with desired micro-organisms for nutrients and reduce the risk of spoilage of the product.

There are phases of growth in micro-organism populations which result in changes in culture conditions.

The phases of growth are the lag phase, the log/exponential phase, the stationary phase, and the death phase.

The lag phase is where enzymes are induced to metabolise substrates.

The log/exponential phase contains the most rapid growth of micro-organisms due to plentiful nutrients.

The stationary phase occurs due to the nutrients in the culture media becoming depleted and the production of toxic metabolites.

Secondary metabolites are also produced in the stationary phase, such as antibiotics.

In the wild, secondary metabolites confer an ecological advantage by allowing the micro-organisms which produce them to outcompete other micro-organisms.

The death phase occurs due to the toxic accumulation of metabolites or the lack of nutrients in the culture.

Growth curves of micro-organisms are often represented using semi-logarithmic scales.

Depending on whether a viable or total cell count is taken, the data resulting will change.

Viable cell counts involve counting only the living micro-organisms

Total cell counts involve counting viable and dead cells.

Only viable cell counts show a death phase where cell numbers are decreasing.